The British Rail Class 55, also known as a Deltic, or English Electric type 5, is a class of diesel locomotive built in 1961 and 1962 by English Electric for British Railways. They were designed for the high-speed express passenger services on the East Coast Main Line (ECML) between Edinburgh and London King's Cross.

They gained the name "Deltic" from the prototype locomotive, DP1 Deltic (the running number DP1 was never carried), which in turn was named after its Napier Deltic power units. At the time of their introduction into service, the Class 55s were the most powerful single-unit diesel locomotives in the world, with a power output of 3,300 hp. They had an official maximum speed of 100 miles per hour, however this was frequently exceeded, especially in their later years of service, with speeds of up to 117 miles per hour, being recorded on level gradients, and up to 125 miles per hour whilst descending Stoke Bank.

Twenty-two locomotives were built, which were used for express passenger services on the ECML, particularly from London to Leeds and Edinburgh. From 1978 the "InterCity 125" High Speed Trains (HSTs) were introduced, and the Deltics were subsequently relegated to semi-fast services on the Kings Cross to York, Edinburgh and Hull routes and continued on sleeper services along the ECML. All were withdrawn from service between January 1980 and December 1981. All remaining examples now preserved.